Palbociclib in highly pretreated metastatic ER-positive HER2-negative breast cancer

We aimed to investigate the role of palbociclib, a first-in-class cyclin-dependent kinase 4 and 6 inhibitor, in postmenopausal women with highly pretreated endocrine therapy-resistant metastatic breast cancer (MBC). Between 28 September 2015 and 14 March 2017, a compassionate use program was established in the University Hospitals Leuven in which 82 postmenopausal women with estrogen receptor-positive, HER2-negative MBC were included after at least four lines of systemic treatment. The efficacy and safety analysis was performed in 82 patients who had received at least one dose of palbociclib and who had at least 6-month follow-up at the data cut-off point. The primary objective was the evaluation of efficacy of the combination of palbociclib and endocrine therapy with clinical benefit as primary endpoint, defined as the absence of progressive disease and being on treatment for at least 6 months. Secondary objectives were the evaluation of toxicity and the identification of potential predictors for clinical benefit. The median age of the patients was 67.1 years (range 34.8-85.9) at the time of inclusion. The average duration of treatment was 5.6 months (range 1-19), with a median progression-free survival of 3.17 (95% CI 2.76-4.70) months. At the data cut-off point, 10 patients were still on treatment with palbociclib. In this highly pretreated setting, 34 patients experienced no progressive disease within 6 months, resulting in an overall clinical benefit rate (CBR) of 41.5%. 20.7% (17/82) showed stable disease for 9 months and 13.4% for 12 months. None of the investigated predicting factors were significantly associated with clinical benefit at 6 months. For 43.9% of the patients, treatment delay or dose reduction was indicated. Palbociclib in combination with endocrine therapy shows an unexpectedly high CBR and favorable safety profile in heavily pretreated endocrine-resistant estrogen receptor-positive, HER2-negative MBC patients.